Use Flexible AFE, Motion Control, and Authentication ICs to Design Point of Care Diagnostics Systems

The trend toward point-of-care (PoC) medical testing is migrating from the laboratory and into the physician’s office, clinic, or even the home. This migration has the potential to accelerate diagnoses, leading to faster patient care, improved outcomes, and lower costs.

Implementing PoC begins with versatile, application-optimized ICs with advanced analog front-ends (AFEs) to interface with various biosensors for the necessary data-acquisition measurements. Each IC must address the unique attributes of sophisticated electrochemical, biological, and related measurements, encompassing accuracy, low power, and highly integrated functions. Successful end products are characterized by high performance, as well as flexibility and upgradability to contribute to future-proofing a platform. They must also be supported by smooth and precise motion control and authentication ICs to ensure data accuracy and privacy.

This article examines the shift to PoC and its design implications. It then describes widely used AFE measurement scenarios and introduces flexible solutions from Analog Devices to meet the PoC measurement, motion control, and authentication requirements.

Why now for PoC?

The driving forces for increased PoC testing and sample processing include the demand for faster medical diagnostics to improve individual health outcomes. Regulatory mandates encourage and even demand more tests. There is also a trend toward localized PoC at a clinic or home to minimize patient disruption, expense, and time. Such systems require simple-to-use yet powerful instrumentation to meet these objectives.

For designers of such systems, AFEs, motion control, and authentication ICs are the front-line interface between patient fluids and vitals and the systems needed to capture, record, assess, and report the resultant data from various sensors. They are the building blocks of the electrochemical and optical diagnostic solutions required to provide a measurement engine to complement a wide range of biosensors and chemistries, while enabling a platform that can be upgraded using software (Figure 1).

Figure 1: Analog and related electronics provide the critical interface between a patient’s vital signs and fluids and the associated PoC instrumentation and data systems. (Image source: Analog Devices)

Diverse application-focused ICs address challenges

Some examples clearly illustrate this situation:

Example #1: Optical fluorescence detection (FLD):

This technique allows researchers to study the distribution, localization, and interactions of biological components within cells or tissues, providing detailed insights into the cellular processes and functions that are often invisible with standard light microscopy. It uses fluorescence induced fluorophores, as opposed to optical absorption, scatter, or reflection.

A fluorophore absorbs light at a particular wavelength, exciting some of its electrons to a higher energy state. As the electrons return to their ground state, the fluorophore emits light at a longer, characteristic emission wavelength. This emitted fluorescence is detected and analyzed, providing high-contrast, molecular-level visualization of biological structures.

Advances in LED-plus-photosensor systems offer additional performance and capabilities. ICs such as the MAX86171 (Figure 2, top), an ultra-low-power optical data-acquisition system with transmit and receive channels, are tailored to these applications. Despite their internal complexity, only a few discrete components are needed in an application (Figure 2, bottom).

Figure 2: The MAX86171 multichannel, ultra-low-power, optical data-acquisition system (top) leverages its high level of internal integration to simplify external wiring and the need for passive support components (bottom). (Image source: Analog Devices)

On the transmitter side, the MAX86171 has nine programmable LED-driver output pins connected to three high-current, 8-bit LED drivers. On the receiver side, the IC has two low-noise, charge-integrating front-ends with ambient light cancellation (ALC) circuits, resulting in a high-performance, highly integrated, optical-based data-acquisition system.

For designs requiring fewer optical channels, the MAX86178ENJ+ is an ultra-low-power, clinical-grade vital-sign AFE supporting up to six LEDs and four photodiode inputs.

Note that the figures of merit and priorities for medical applications differ from those for non-medical situations such as optical data channels. As the light levels are usually relatively low, the absolute noise floor of the optical front-ends is the critical parameter rather than the signal-to-noise ratio (SNR).

While bandwidth and sampling rates are very low, and the parameters of interest do not vary at multi-kilohertz rates in the biological world, the complex analog nature of patients and the signals mandates different sets of priorities in specifications. These include high sensitivity, wide dynamic range, and low noise to succeed in a changing operating environment where the patient’s skin and internal organs continually shift to change the contact area and force, even slightly. They also do so in the presence of various types of interfering noise and variations, further complicating matters.

To meet the application requirements, the MAX86171 features a dynamic range between 91 and 110 decibels (dB) depending on test arrangement, a resolution of 19.5 bits, a dark-current noise of less than 50 picoamperes (pA) (RMS), and an ambient light rejection figure of better than 70 dB at 120 hertz (Hz).

Example #2: Potentiometry, amperometry, voltammetry, and impedance measurements:

Electrical engineers are comfortable with measuring voltage, current, and impedance, along with their relationships, by choosing from a wide variety of standard instrumentation. However, these measurements have unique requirements and constraints in a chemical and biological setting and present distinct scenarios:

• Potentiometry: using a potentiostat to measure the electrical potential between two electrodes to determine the concentration of a substance in a solution
• Amperometry: using an amperometric arrangement to detect ions in a solution based on electric current or changes in electric current
• Voltammetry: where a specific voltage profile is applied to a working electrode as a function of time, and the current produced by the system is measured, usually via a potentiostat
• Impedance: measuring the voltage and current relationship of skin and body

To assess these parameters, the AD5940 provides a wide range of functionality and interface options in a 56-ball WLCSP measuring 3.6 × 4.2 millimeters (mm) (Figure 3). This low-power AFE is designed for portable applications that require high-precision electrochemical techniques such as amperometric, voltammetric, or impedance measurements.

Figure 3: The AD5940 AFE incorporates the sophisticated functions needed for precise, low-power amperometric, voltammetric, or impedance measurements. (Image source: Analog Devices)

The AD5940 has two excitation loops and one common measurement channel. The first loop consists of a dual-output string, a digital-to-analog converter (DAC), and a low-noise potentiostat, and it can generate signals from 0 Hz to 200 Hz.

One output of the DAC controls the noninverting input of the potentiostat, and the other controls the noninverting input of the transimpedance amplifier (TIA). The second loop consists of a 12-bit DAC capable of generating excitation signals up to 200 kilohertz (kHz).

On the input side, there is a 16-bit, 800 kilosample per second (kS/s) analog-to-digital converter (ADC) with input buffers, an antialias filter, and a programmable-gain amplifier (PGA). A multiplexer selects input channels for external current and voltage inputs and internal channels for supply voltages, die temperature, and reference voltages.

The current inputs include two TIAs with programmable gain and load resistors for measuring different sensor types. The first TIA measures low-bandwidth signals, while the second TIA measures high-bandwidth signals up to 200 kHz.

Users of ICs that offer this level of integration and versatility benefit from evaluation kits that go beyond the IC. For the AD5940, the EVAL-AD5940BIOZ electrocardiography (ECG/EKG) sensor Arduino platform evaluation expansion board provides a familiar development environment (Figure 4). This kit also helps future-proof designs when new test requirements are added, as the platform can be upgraded via software.

Figure 4: The EVAL-AD5940BIOZ electrocardiography (ECG/EKG) sensor Arduino Platform evaluation expansion board simplifies the challenge of using and assessing the AD5490 when making the subtle, low-level measurements it is designed to provide. (Image source: Analog Devices)

Each AD5940 evaluation board targets a specific end-application measurement objective. The Arduino-like board configures and communicates with the AD5940 through the SPI peripheral. A graphical user interface (GUI) tool for measurements with graphing and data-capture capabilities is available for initial evaluation. Many example projects written in embedded C include instructions on how to set the programming environment and run the examples.

Example #3: Data authentication:

Data stored in diverse locations and transmitted using wireless near-field communication (NFC) links raises issues related to data authenticity and even the risk of reuse, misuse, and counterfeit samples or cartridges.

To address these concerns, the MAX66250 Secure Authenticator (Figure 5, top) provides robust countermeasures, with all stored data cryptographically protected from discovery. It is compatible with NFC-enabled embedded systems (Figure 5, bottom), where the risk of unauthorized access is higher.

Figure 5: The MAX66250 Secure Authenticator (top) provides multiple levels of advanced data security and authentication support; it also incorporates an NFC interface (bottom) for wireless data transfer. (Image source: Analog Devices)

The secure authenticator combines FIPS 202-compliant Secure Hash Algorithm (SHA-3) challenge-and-response authentication with secured EEPROM. The device provides a core set of cryptographic tools derived from integrated blocks, including an SHA-3 engine, 256 bits of secured user EEPROM, a decrement-only counter, and a unique 64-bit ROM identification number (ROM ID). The unique ROM ID is a fundamental input parameter for cryptographic operations and serves as an electronic serial number within the application. The device communicates over an RF interface compliant with ISO/IEC 15693.

For wired links, the DS28E16Q+U 1-Wire Secure SHA-3 Authenticator provides the same cryptographic tools as the MAX66250, including a unique ROM ID.

Example #4: Motion/motor control:

Many PoC devices and stations require carefully controlled motion to convey a test strip or test tube between stations, combine and transfer reagents, or add or release precise amounts of liquids and perform pipetting. These applications often require precise micro-stepping and smooth stop, start, and ramp generation to provide high-resolution and vibration-free movement for quick, precise, reliable, quiet, reproducible, and energy-efficient motion.

The Trinamic TMC5072-LA-T single/dual-channel stepper-motor controller and driver IC (Figure 6, top) with serial communication interfaces is suited to these applications. When wired for parallel operation, it offers a coil-current drive capability of 1.1/1.5 amperes (A) peak per motor and 2.2/3 A peak for one motor.

For basic operation, the companion TMC5072-BOB evaluation kit (Figure 6, bottom) includes an onboard TMC5072 and connects to an Arduino Mega using a single-wire universal asynchronous receiver/transmitter (UART). A graphical user interface (GUI) provides tools for easily setting parameters, visualizing data in real time, and developing and debugging stand-alone applications.

Figure 6: The TMC5072-LA-T single/dual-channel stepper-motor controller and driver IC (top) provides precision performance and smooth operation; it is supported by the TMC5072-BOB eval kit (bottom). (Image source: Analog Devices)

The TMC5072 combines flexible ramp generators for automatic target positioning and offers noiseless operation, maximum efficiency, and high motor torque. The 7 × 7 mm IC offers additional advanced features:

• StealthChop for extremely quiet operation and smooth motion
• SpreadCycle highly dynamic motor-control chopper
• DCStep for load-dependent speed control
• StallGuard2 high-precision sensorless motor-load detection
• CoolStep current control for energy savings up to 75%

Of course, a single motion-control device is not optimal for all PoC system needs, no matter how feature-laden and function-rich. For this reason, Analog Devices offers a wide range of motor-related ICs and support functions for PoC, including:

• TMC4671-LA: integrated servo controller providing field-oriented control (FOC) for brushless DC/permanent magnet synchronous motors (BLDC/PMSMs) and 2-phase stepper motors
• TMC4671-LEV-REF: reference design for TMC4671 with BLDC servo driver
• TMC5240ATJ+T: smart, high-performance stepper-motor controller and driver IC with serial communication interfaces (single-axis version of the TMC5072)
• TMC4361A-LA-T: motion controller for stepper-motor drivers, offering S-shape ramping in fast and jerk-limited motion-profile applications
• TMC2240ATJ-T: smart integrated stepper driver with step/direction and SPI interfaces.

Conclusion

A combination of factors is driving many medical tests and assessments toward a more localized, fast-response PoC model. Highly integrated, application-focused ICs such as AFEs, motion control, and authentication enable this trend. Analog Devices offers many choices of high-performance, low-power devices that are optimized for these applications that meet the technical and regulatory requirements. They also provide the flexibility and upgradability required of a future-proof platform.

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• Point of Care (PoC) Diagnostic Solutions
• Results in Minutes: How Rapid Point of Care Testing (POCT) is Transforming Diagnostics
• How Integrated Optical Receivers Future-Proof Point-of-Care Instruments
• Jack of All Trades in Impedance Measurement
• AN-1563: Optimizing the AD5940 for Electrochemical Measurements
• Trinamic’s Stepper Motor Control Module Helps Combat Malaria
• High-Performance Lab Automation Solutions for Close Tolerances
• How Secure Electronic Authentication Mitigates Risk at the Point-of-Care